Xerogels and aerogels are highly porous materials with a particularly low envelope density and high surface area. They typically also display exceptionally low thermal conductivity and acoustic propagation properties. As such, they are useful in a wide range of applications including as purification/separation media, non-reflective panels, gas storage media, catalyst support, porous substrates e.g. sponges and electrochemical device electrodes (for supercapacitors, fuel cells and lithium ion batteries).
The most common examples are silica aerogels usually made by sol-gel processes and carbon hydrogels obtained from pyrolysis of resorcinol-formaldehyde resin.
Graphene is a single sheet of carbon atoms patterned in a honeycomb lattice form. Graphene has recently attracted much attention for its unique electronic properties, excellent mechanical properties, and superior thermal properties (K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666-669 and A. K. Geim, Science, 2009, 324, 1530-1534). Attempts to exploit these properties in macroscopic form depend on the development of appropriate processing techniques.
Graphene aerogel with high electrical conductivity (1×102 S m−1) has been synthesised by sol-gel polymerization of resorcinol (R) and formaldehyde (F) with sodium carbonate as a catalyst (C) in an aqueous suspension of graphene oxide (GO) (M. A. Worsley, P. J. Pauzauskie, T. Y. Olson, J. Biener, J. H. Satcher, T. F. Baumann, J. Am. Chem. Soc., 2010, 132, 14067-14069)
Ion linkages have also been applied for the preparation of 3D architectures of graphene (Z. H. Tang, S. L. Shen, J. Zhuang and X. Wang, Angew. Chem., Int. Ed., 2010, 49, 4603-4607; X. Jiang, Y. Ma, J. Li, Q. Fan and W. Huang, J. Phys. Chem. C, 2010, 114, 22462).
Graphene oxide sponges were synthesised by vacuum centrifugal evaporating system (F. Liu, T. S. Seo, Adv. Funct. Mater., 2010, 20, 1930-1936).
Graphene hydrogel has been prepared by an hydrothermal process under high pressure, and the obtained hydrogel is electrically conductive, mechanically strong, and exhibits a high specific capacitance (Y. X. Xu, K. X. Sheng, C. Li, G. Q. Shi, ACS Nano, 2010, 4, 4324-4330).
3D architectures of graphene have been fabricated via an in situ self-assembly of graphene obtained by mild chemical reduction of graphene oxide in water under atmospheric pressure (W. Chen, L. Yan, 2011, Nanoscale, 3, 3132-3137).
Therefore, the present invention seeks to provide a method of obtaining cross-linked graphene and graphene oxide networks, which are selected from aerogels and xerogels. The present invention also seeks to provide cross-linked graphene networks which are selected from aerogels and xerogels which allow more control over the density, shape, conductivity and internal surface of the graphene, so that they display desirable electrical and mechanical properties.
The present invention also seeks to utilise such electrically heatable graphene and graphene oxide aerogels/xerogels which have high surface area, and are highly inter-connected to provide robust networks that are attractive for applications such as filtration, sorption, gas storage and catalyst support.